The present invention relates to liquid filtration systems having centrifugal vacuum producers to maintain sub-atmospheric pressures in an enclosed space beneath a filter media from which solid contaminants are periodically removed. More specifically the invention relates to improved controls for the electric motors driving the vacuum producer and the means effecting contaminant removal in such filtration systems.
In many industrial processes, liquids are used for cooling and/or lubrication purposes in machining operations, and the like. The contaminated liquid is conducted from the area of the operation wherein it is utilized to filtration apparatus for removal of the solid foreign matter and the clean liquid is returned for further use. Although the process is essentially continuous in terms of circulation of the liquid to and from the area of use and the filtering apparatus, in most typical operations it will be found that the flow of liquid to and through the filtering apparatus, and thus the level of contaminated liquid within the apparatus, is subject to fluctuation.
A type of filtering apparatus commonly employed in applications such as just described includes a tank wherein the contaminated liquid is deposited, separated from an underlying compartment by filter means overlying an opening through which the tank communicates with the compartment. Solid particles are collected on the upper side of the liquid permeable media, forming a contaminated bed or "filter cake", which is periodically removed when its resistance to flow becomes too great to maintain at least a minimum desired flow rate of the liquid into the lower compartment.
The gravity-induced flow of liquid through the media is enhanced in such filtration apparatus by creating a sub-atmospheric pressure in the lower compartment, which is normally sealed on its uppeer side by the pool of liquid in the tank. However, as previously mentioned, the flow rate of contaminated liquid to the tank is subject to fluctuation, and the rate of flow through the media varies in accordance with the degree of filter cake build-up. Thus, when the filter cake is removed from the tank there is less resistance to flow and the level of liquid in the tank normally drops, sometimes to the point that it no longer entirely seals the opening between the tank and lower compartment.
The sub-atmospheric pressure in the lower compartment, which is therefore commonly termed a vacuum chamber, is provided by a centrifugal vacuum producer driven by an electric motor to remove air from the vacuum chamber. This type of vacuum producer will seek an equilibrium between air volume or flow rate and the sub-atmospheric pressure in the vacuum chamber. When the vacuum chamber is not sealed, as a result of the pool of liquid in the tank dropping below a certain level, air is free to enter the chamber and the vacuum producer will operate at a fast rate, demanding a high horsepower input to the electric motor.
Since the required horsepower increases with air flow, to cover all conditions of operations requires large horsepower motors which are utilized to full capacity only at infrequent intervals. On the other hand, if the vacuum producer is driven by an electric motor having a horsepower rating in line with that required for normal operation when the vacuum chamber is essentially sealed by the pool of liquid in the tank, the motor would become overloaded when the liquid level drops to or below a certain level. One present means of dealing with this problem is to place a fixed restriction in the line between the vacuum changer and vacuum producer to limit the rate at which air will be withdrawn when the liquid level in the tank is too low to maintain the sealed condition of the vacuum chamber. Although the restriction may be adjustable, it obviously cannot provide an optimum balance between the rate of air removal from the vacuum chamber and the horsepower demand on the vacuum producer motor under all conditions of operation.
The filter cake is removed by a second electric motor which moves either a liquid permeable membrane (disposable or reuseable) or an apron type conveyor which scrapes the contaminants from the surface of a stationary media or strainer, for example, the type commonly known as "wedge-wire." This motor is intermittently operated in response to signals which are in some manner related to the degree of filter cake build-up on that portion of the media which is in filtering position. For example, some filtration systems of this type utilize switches responsive to the pressure differential above and below the media for starting and stopping the contaminant removal drive motor since this differential increases in proportion to the resistance to liquid flow provided by the filter cake. Other systems employ switches responsive to the level of liquid in the upper compartment, which rises as the filter cake builds up and increasingly restricts the flow of liquid through the media. In any case, the controls will have a certain deadband, which is usually fixed once the system has been installed, for starting and stopping the motor at certain spaced values of pressure differential, liquid level, etc. The upper and lower limits, i.e., the deadband, of commonly available controls of this type may produce undesirably wide fluctuations in filtering efficiency.
It is a principal object of the present invention to provide novel and improved liquid filtration apparatus having single element movable in response to the level of contaminated liquid in an upper tank to control both starting and stopping of the drive motor for removing contaminants from the tank and the horsepower demanded of the vacuum producer motor which evacuates air from a lower, enclosed compartment which receives the filtered liquid.
Another object is to provide a control system for the contaminant removal drive motor in a liquid filtration system which may be quickly and easily adjusted to provide starting and stopping of the motor at any desired levels of contaminated liquid in the receiving tank.
An additional object is to provide, in a liquid filtration system having upper and lower compartments separated by a filtering media and a vacuum producer for evacuating air from the lower compartment to maintain a sub-atmospheric pressure therein, novel means for preventing overloading of the vacuum producer motor during conditions of normally high operating demands without interruption in operation.
Another object is to provide means responsive to the level of contaminated liquid in filtration equipment for controlling the volume of air handled by apparatus for producing sub-atmospheric pressures in the filtered liquid compartment to prevent overloading of an electric motor driving such apparatus.
A further object is to provide novel and improved means for varying the volume of air pumped by a vacuum producer from the vacuum chamber of liquid filtration apparatus having an open-topped contaminated liquid tank separated from the vacuum chamber by a layer of fluid-permeable filter media.
A still further object is to provide apparatus for preventing the overloading of an electric motor used in liquid filtration equipment which conveniently incorporates elements for performing other functions in connection with operation of such equipment.
In a more general sense, the object of the invention is to provide means which improve the operation of liquid filtration apparatus, allowing the vacuum producer to operate with a smaller motor while maintaining its required performance, and permitting selective "fine-tuning" of the media drive motor controls.
Other objects will in part be obvious and will in part appear hereinafter.